Question: Four years ago you invested some money at $10\%$ interest. You now have $\$439.23$ in the account. If the interest was compounded yearly, how much did you invest 4 years ago?
Solution: Let $x$ be the initial amount.  After four years, at a ten percent annual interest rate, the investment will have grown to $x \cdot 1.1^4$, so $x \cdot 1.1^4 = 439.23$.  Then $x = 439.23/1.1^4 = \boxed{300}$.